Over the past twenty years, rare earth (RE) doped optical fibers have had a tremendous impact on the laser industry. The first application that deployed a significant volume of rare earth doped optical fiber based devices was optical amplification in the telecommunications industry. In this application, optical signals carrying data are sent through single mode fiber typically at a wavelength near 1.5 μm. As these signals propagate over long distances, they are attenuated due to scattering and absorption losses in the transmission fiber. By coupling these transmission fibers to a section of erbium doped fiber that is pumped with a wavelength near 980 nm or 1480 nm, these signals can be amplified back to their original intensity level. These devices are commonly known as erbium doped fiber amplifiers (EDFAs).
When compared with other lasers and optical amplifiers, fiber based devices typically offer higher gain and higher overall efficiency. As the average power levels, pulse energies and peak powers of fiber lasers and fiber amplifiers continue to increase, rare earth doped optical fibers have begun to be used in a far broader range of applications. These applications are found in the medical, industrial, defense, and semiconductor processing industries.
Increasing the average power of fiber lasers can be limited by the brightness of laser diode pumps, the ability to couple power into fiber, and nonlinear effects caused by high optical power. These issues can be effectively addressed using fibers with large core sizes.
The fundamental transverse mode of an optical fiber has very desirable characteristics in terms of beam shape, minimum beam expansion during propagation through free space (often referred to as “diffraction limited”) and optimum focusability. As a result, most applications benefit greatly from single mode, or fundamental mode operation of fiber lasers and amplifiers. As the core size of an optical fiber is increased to enable greater pulse energies and higher peak powers, the fiber begins to support the propagation of more than one transverse optical mode. The number of modes supported by an optical fiber can be roughly calculated by using the fiber's so-called V-number. The V-number of a fiber is defined as V=2πa/λNA, wherein a is the radius of the fiber core and NA is the numerical aperture of the core. The number of modes supported by the fiber is then given by roughly one half the square of the V-number. It can be shown that a fiber with a V-number less than about 2.4 supports the propagation of only the fundamental mode.
Prior methods of increasing the peak and average powers of multimode amplifiers are described in Fermann et al., U.S. Pat. No. 5,818,630, which is incorporated herein by reference. A diffraction limited seed source is optically coupled to a multi-mode fiber amplifier. Through the use of a mode-converter, defined as either a set of bulk lenses or a tapered section of fiber, the beam size is changed to match as nearly as possible that of the fundamental mode of the optical fiber. If this is done well and the fiber is not disturbed, this approach can result in near fundamental mode operation of a multimode fiber amplifier. However, for the following three reasons, this approach has limited utility in practical applications. First, most seed lasers cannot be effectively coupled into only the fundamental mode of a multimode fiber. Even if the seed laser is a single transverse mode laser, unless the seed laser is a fiber laser, the fundamental mode of the seed laser is not the same as the fundamental mode of an optical fiber. For this reason, even with such a mode converter, higher order modes of the multimode optical fiber will be excited to some extent. Further, any changes in launch conditions due to, for example, movement or temperature changes can alter coupling of seed power into each of the numerous optical modes of the fiber. This causes corresponding changes in output beam shape and mode quality. In addition, when higher order modes are excited in a multimode fiber, the output beam shape and mode quality is highly sensitive to both micro and macro bends in the fiber. The presence of higher order modes can also result in poor beam pointing stability. Even if a stable package could be developed to prevent changes in micro and macro bending of the fiber during operation, this bend sensitivity makes manufacturing challenging as the output is not stable. These limitations largely render this amplifier configuration impractical for most commercial applications when not coupled with other mode control techniques.
In other approaches, tightly coiled fibers are used to suppress higher order modes. This approach results in a distributed bend induced loss that strips the power from the higher order modes in the amplifier. The induced loss is a relatively strong function of the spatial order of the fiber mode. For modes that are radially symmetric, the loss is independent of the axis of the coil with higher order modes experiencing higher loss. For modes that are radially asymmetric, the loss is dependent on the axis of the coil. To ensure sufficient loss for all modes, it is therefore sometimes required to coil the fiber about one axis follow by a coil on an axis oriented at 90 degrees with respect to the first one. Amplifiers made in this way can be designed to operate stably in only the fundamental mode.
Unfortunately, there are also other practical limitations associated with this design as well. One limitation is that to strip the higher order modes effectively, loss is also created for the fundamental mode. This distributed loss for the fundamental mode potentially limits the overall efficiency of the fiber amplifier. Another limitation is that as the fiber is bent, the effective mode area of the fundamental beam is reduced. This increases the irradiance of the signal within the fiber and, as described earlier, leads to increased amounts of nonlinear effects. These nonlinear effects ultimately limit the peak power capability of the fiber amplifier.
A significant improvement is described by Filippov et al., U.S. Patent Application Publ. 2010/0247047, disclose using a single-mode fiber as the rare earth doped amplifying region in a continuous wave (CW) laser cavity. A high beam quality can be achieved based on a taper having a smallest cross sectional dimension that permits only single mode propagation. This approach, although demonstrated to have a high beam quality, lacks in efficiency and power scaling capability for the following reasons. The single-mode portion at the smaller end of the taper restricts the core size to a value enabling a V-number of less than 2.4. In order to increase the peak and average power with high efficiency (i.e. to not lose pump power due to vignetting) the corresponding pump waveguide, i.e., the cladding size of the single-mode portion is restricted by the available pump brightness at the required pump power level. Therefore to increase the peak and average power, the single-mode cladding diameter needs to be increased. This leads to a lower core/cladding area ratio, which subsequently reduces pump absorption and makes the amplifier longer, thus inherently lowering the nonlinear threshold of the system, preventing further power scaling of high peak power, high average power, and/or spectrally narrow pulses.
In view of the above, further improvements in peak and average power of pulsed fiber lasers require an improved waveguide design.